Speed up notification pullup & pulldown

based on an idea by Mr.Nigma @xda:
http://forum.xda-developers.com/showthread.php?t=2267307

Change-Id: I61213058b5cedddf90e974713af05c038e3af039
Add method to PowerManager that allows querying the lowest possible
brightness.

Used by the auto-brightness adjustment to adjust the sliders.